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Electrical Signaling Through Changes In Membrane Potential
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19
Physiology
05/05/2012

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Term
excitable tissues
Definition
nerve and muscle tissue

have the ability to produce rapid, transient changes in their resting membrane potential when 'excited.' Excitable tissues can also conduct these electrical signals. These rapid fluctuations in the membrane potential serve as electrical signals.
Term
changes in resting membrane potential
Definition

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brought about by changing the membranes permeability to various ions. By increasing or decreasing the membrane's permeability to a particular ion, the resting membrane potential moves toward or away from that ion's equilibrium potential (per the Goldman equation)

Term
polarized
Definition
cells are said to be polarized because of the separation of charge. A typical resting membrane potential in a neuron is -70 mV.
Term
depolarization
Definition
occurs when the cell's potential moves in the positive direction
Term
repolarization
Definition
describes the movement of a cell's potential back toward its resting potential
Term
hyperpolarization
Definition
occurs when the cell membrane potential moves further negative away from its resting membrane potential
Term
graded potential
Definition
demonstrate the following characteristics:
--proportional in size to the triggering stimulus
--decremental conduction (the change in membrane potential decreases as it moves away from the site of stimulation)
--summation (the effects of graded potentials can be added together in temporal or spatial summation)
--excitatory or inhibitory

triggering stimulus may be either a neurotransmitter (via ligand gated channels) or a sensory stimulus (e.g. touch via mechanically gated channels, light on photoreceptors, chemical changes detected by photoreceptor). These types of triggering events result in an alteration of ion movement across the cell membrane--i.e. a graded potential.

A graded potential of sufficient magnitude may lead to the generation of an action potential.
Term
temporal summation
Definition
if stimuli occur in rapid succession (before the events associated with the previous stimuli disappear), the effects add together. The closer together in time, the greater the effect
Term
spatial summation
Definition
stimuli occurring in the same location at the same time will add together
Term
excitatory or inhibitory effects of stimulation
Definition
the direction of change, whether a graded potential is excitatory or inhibitory, is dependent upon which ion channels are opened or closed

excitatory graded potentials manifest as depolarization (inside of the cell becomes more positive). Excitatory potentials may result from an increase in Na+ permeability, which allow more (+) ions into the cell

Inhibitory graded potentials manifest as hyperpolarization (inside of the cell becomes more negative). Inhibitory potentials often result from an increase in K+ or Cl- permeability, making the inside of the cell more (-).
Term
action potential
Definition

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result when a graded potential reaches threshold

 

display the following characteristics:

--changes in membrane potential caused by time dependent opening/closing of voltage gated channels

--always excitatory

--do not summate (due to having refractory period)

--action potentials demonstrate an all or none response. When threshold has been reached or exceeded, the amplitude of an action potential is the same.

--do not travel decrementally

Term
threshold stimulus
Definition
minimum stimulus required to cause the voltage gated Na+ channels to open is one which depolarizes the cell ~15 mV above its resting potential

will result in the sequence of permeability changes that create an action potential
Term
subthreshold stimulus
Definition
creates a graded potential that does not reach threshold, and therefore will not result in an action potential
Term
suprathreshold stimulus
Definition
a stimulus that results in a depolarization greater than threshold
Term
voltage gated Na+ channels
Definition
When graded potentials reach a threshold, voltage gated Na+ channels are activated, which in turn allow for the rapid influx of Na+ into a cell down its electrochemical gradient. With the entrance of Na+, the cell becomes more (+), which in turn causes the opening of more voltage gated Na+ channels. (positive feedback)

contain 2 gates (activation and inactivation gate) activation gate initially opens in response to depolarization deactivation gate which begins to close about 1 msec after depolarization. Once closed, the inactivation gate prevents movement of Na+ into the cell. It remains closed until the cell returns to resting membrane potential.

 

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Term
voltage gated K+ channels
Definition
triggered to open with depolarization, however the open much more slowly than the voltage gated Na+ channels. These K+ channels do not completely open until the time the inactivation gate on the Na+ channels close. At this point, the potential inside of the cell is +30 mV. There is a strong electrochemical gradient now favoring movement of K+ out of the cell, which results in repolarization of the cell. As the cell repolarizes, the stimulus for opening the K+ channels weakens, and they slowly close. Because they are slow to close, an excess of K+ leaves the cell, resulting in hyperpolarization.
Term
refractory period
Definition

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period of reduced excitability during and immediately after an action potential. As a result, there is no summation with action potentials. Refractory periods also ensure one way transmission of signals absolute and relative refractory periods

Term
absolute refractory period
Definition
coincides with the time period when Na+ channels are either already open and will not be affected by a second stimulus, or whe inactivation gates are closed and incapable of opening. This time period encompasses all of depolarization and repolarization phases.
Term
relative refractory period
Definition
occurs immediately after the absolute refractory period. It is possible to generate an action potential, however it takes a stronger stimulus. This relative refractory period is primarily due to the increased permeability of K+ that continues past repolarization into hyperpolarization.